The somatolactogenic hormones prolactin (PRL) and growth hormone (GH) are necessary for the full growth and maturation of vertebrate species.
Prolactin (PRL) was originally identified as a neuroendocrine hormone of pituitary origin. PRL expression has also been detected in the decidua, breast and T-lymphocytes (Clevenger, C. V. and Plank, T. L. J. Mammary Gland Biol. Neoplasia 1997 2:59–68; Mershon et al. Endocrinology 1995 136:3619–3623; DiMattia et al. Endocrinology 1986 122:2508–2517; Ginsburg, E. and Vonderhaar, B. K. Cancer Res. 1995 55:2591–2595; Gellersen et al. Mol. Endocrinol. 1994 8:356–373; Clevenger et al. Proc. Natl. Acad. Sci. USA 1990 87:6460–6464; Montogomery et al. Biochem. Biophys. Res. Commun. 1987 145:692–698). A primary function of this hormone lies within the breast. However, functional pleiotropism of this peptide with regard to reproduction, osmoregulation and behavior has also been recognized (Nicoll, C. S. Handbook of Physiology; Section 7: Endocrinology, pp. 253–292, Washington, D.C.: American Physiology Society. 1974). Several lines of evidence have also indicated an immunoregulatory role for this peptide (Clevenger et al. Journal of Endocrinology 1998 157:187–197; Weigent, D. A. Pharmacol. Ther. 1996 69:237–257). Structural analysis of PRL has revealed it to be related to members of the cytokine/hematopoietin family which also includes growth hormone, erythropoietin, granulocyte-macrophage colony stimulating factor (GM-CSF) and interleukins 2–7 (Bazan, J. F. Immunol. Today 1990 11:350–354)
The pleiotropic actions of PRL are mediated through its receptor (PRLr), a member of the superfamily of type I cytokine receptors. PRLr is present on numerous tissues including mammary epithelia, T and B lymphocytes and macrophages (Dardenne et al. Endocrinology 1994 134:2108–2114; Pellegrini et al. Mol, Endocrinol. 1992 6:1023–1031). Acting through its receptor, PRL signaling stimulates cell proliferation, survival and cellular differentiation in a tissue- and microenvironment-dependent manner. In the mammary and immune systems, PRL is believed to act at the endocrine, paracrine, and autocrine levels in regulating T-lymphocyte proliferation and survival (Gala, R. R. PSEBM 1991 198:513–527; Yu-Lee, L. Y. Proceedings of the Society for Experimental Biology and Medicine 1997 215:35–52; Kooijman et al. Adv. Immunol. 1996 63:377–454; Prystowski, M. B. and Clevenger, C. V. Immunomethods 1994 5:49–55) and the terminal maturation of mammary tissues (Kelly et al. Rec. Prog. Horm. Res. 1993 48:123–164; Shiu et al. Rec. Prog. Horm. res. 1987 43:277–289). PRL is also believed to act as both an endocrine and autocrine/paracrine progression factor for mammary carcinoma in both rodents and humans (Welsch, C. W. Cancer Res. 1985 45:3415–3443; Welsch, C. W. and Nagasawa, H. Cancer Res. 1977 37:951–963; Manni et al. Cancer Res. 1986 37:951–963; Malarkey et al. J. Clin. Endocrinol. Metab. 1983 56:673–677; Clevenger et al. Am. J. Pathol. 1995 146:1–11; Fields et al. Lab. Invest. 1993 68:354–360; Ormandy et al. J. Clin. Endocrinol. Metab. 1997 82:3692–3699; and Mertani et al. Int. J. Cancer 1998 79:202–22).
Pleiotropic actions of GH are also largely mediated through a type I cytokine receptor, GHr.
Ligand-induced dimerization of PRLr and GHr activates several associated signaling cascades including the Jak-Stat, Ras-Raf, and Fyn-Vav pathways (Campbell et al. Proc. Natl. Acad. Sci. USA 1994 91:5232; Clevenger et al. J. Biol. Chem. 1994 269:5559; Clevenger et al. Mol. Endocrinol. 1994 8:674; Clevenger et al. J. Biol. Chem. 1995 270:13246). However, studies indicate that both PRL and GH are internalized via an endosomal-like pathway and transported across the endoplasmic reticulum (ER) and nuclear envelopes (Clevenger et al. Endocrinology 1990 127:3151; Rao et al. J. Cell Physiol. 1995 163:266). This process is referred to as nuclear retrotranslocation. The mechanism of this retrotranslocation, and the nuclear action of these somatolactogenic hormones, however, is not well understood.
Both PRL and GH lack enzymatic activity. These hormones also contain no nuclear translocation signal. Thus, for PRL and GH to act within the nucleus, they must do so through a binding partner or chaperone.
CypB is a member of the cyclophilin family of cistrans peptidyl prolyl isomerases (PPI) (Price et al. Proc. Natl. Acad. Sci. USA 1991 88:1903; Ruhlmann, A. and Nordheim, A. Immunobiol. 1997 198:192; Resch, K. and Szamel, M. Int. J. Immunopharmac. 1997 19:579). This family of proteins was initially identified as the binding partners for the immunosuppressive agent cyclosporine (CsA). CsA interacts with the cyclophilin with high affinity, inhibiting their PPI activity and the action of the phosphatase calcineurin, necessary for NF/AT-transactivated expression of IL-2 (Kronke et al. Proc. Natl. Acad. Sci. USA 1984 81:5214; Liu et al. Cell 1991 55:807; Friedman, J. and Weissman, I. Cell 1991 66:799; McCaffrey et al. J. Biol. Chem. 1993 268:3747; Bram, R. J. and Crabtree, G. R. Nature 1994 371:355; Bram et al. Mol. Cell Biol. 1993 13:4760). Structurally CypB is a β-barrel protein containing both N-terminal ER-leader and putative nuclear translocation signal sequences and C-terminal ER-retention sequences (Allain et al. J. Immunol. Meth. 1995 178:113; Mariller et al. Biochem. Biophys. Acta 1996 1293:31). CypB has been observed in the ER and nucleus, and can be found in appreciable levels in blood (150 ng/ml) and breast milk (Hirada et al. Cell 1990 63:303; Price et al. Proc. Natl. Acad. Sci. USA 1994 91:3931). Cyclophilins, via their PPI activity, facilitate protein folding and have been shown to contribute to the maturation of several proteins, including carbonic anhydrase and the HIV glycoprotein Gag (Taylor et al. Prog. Biophys. Molec. Biol. 1997 67:155; Streblow et al. Virology 1998 245:197). Despite these insights, the physiologic function of CypB has remained uncertain.
It has now been found that cyclophilin B (CypB) interacts specifically with somatolactogenic hormones, PRL and GH, as a chaperone mediating the transport, maturation and/or function of these proteins.